1. Field of the Invention
The present invention relates to a digital subscriber line communicating system which utilizes an existing telephone line as a high speed data communication line. More particularly, it relates to an improvement of a modulation/demodulation system in a transmission apparatus used in the above-mentioned transmission system.
In recent years, multimedia services such as internet and so forth have expanded through the whole society including usual homes. Accompanied by such development, it has been strongly required to promptly provide an economical and reliable digital subscriber line communicating system for utilizing such services.
2. Description of the Related Art
(1) An Explanation of an ADSL
As a technique to provide a subscriber line communicating system which utilizes the existing telephone line as a high speed data communication line, an xDSL (Digital Subscriber Line) is known. xDSL is a communicating system which utilizes a telephone line and a modulation/demodulation technique. xDSLs are generally classified into a symmetric type and an asymmetric type. In the symmetric type, upstream transmission speed from a subscriber home (hereinafter referred to as a remote terminal side) to an accommodating central office (hereinafter referred to as a central office side) is symmetric with the transmission speed from the central office side to the remote terminal side. In the asymmetric type, the transmission speed from the remote terminal side to the central office side is asymmetric with the transmission speed from the central office side to the remote terminal side.
In the asymmetric xDSLs, there is an Asymmetric DSL (ADSL) modem which is provided with the G.DMT standard having a downstream transmission speed of about 6 Mbit/sec. and the G.lite standard having a downstream transmission speed of about 1.5 bits/sec. Both of the G.DMT and G.lite employ Descrete Multitone (DMT) modulation.
(2) An Explanation of the DMT Modulation
DMT modulation will be explained using G.lite as an example. This explanation and the associated drawing will describe only the downstream modulation/demodulation from the central office to the remote terminal. However, DMT modulation is also possible in the upstream modulation/demodulation.
Firstly, transmitting data is input into an ADSL transceiver unit (ATU) in the central office and a non-symbol time (¼ kHz) of the data is stored in a serial to parallel buffer. The stored data are divided into a plurality of groups. A predetermined number of transmission bits per carrier signal is previously allocated to each group in accordance with a transmitting bitmap which will be described later in detail. Each group is output to an encoder. In the encoder, each group of the input bit series is converted into a signal point expressed by a complex number for an orthogonal amplitude modulation and is output to IFFT. The IFFT performs the conversion from each of the signal points to transmit the signal sequences by an inverse fast Fourier transform. The signals from the IFFT are output to a parallel to serial buffer. Here the sixteen points of the outputs of the IFFT are added as a Cyclic Prefix to the head of each DMT symbol. The output of the parallel to serial buffer is supplied to a D/A converter in which the digital signal with a sampling frequency of 1.104 MHz is converted into an analog signal. The analog signal is transmitted through a metalic line to a remote terminal.
At the remote terminal side, the analog signal is converted into a digital signal with the sampling frequency of 1.104 MHz by an A/D converter. Each DMT symbol of the digital signal is stored in a serial to parallel buffer. In the buffer, the Cyclic Prefix is removed from the digital signal, and the remaining signal is output to an FFT. In the FFT, a fast Fourier transform is effected to generate or demodulate the signal points. The demodulated signal points are decoded by a decoder in accordance with a receiving bitmap having the same values as those in the transmitting bitmap. The decoded data are stored in a parallel to serial buffer as receiving data of bit-sequences.
(3) A Detailed Explanation of the Bitmap
The bitmap described in the explanation of the DMT will be explained in detail with reference to FIGS. 13A and 13B.
The apparatus at the central office side and the apparatus at the remote terminal side both measure the ratio of the receiving signal to noise (hereinafter referred to as S/N) during a training period prior to communication to determine the number of bits to be transmitted by each modulating carrier. As shown in FIGS. 13A and 13B, for a carrier signal with a larger S/N, a larger number of bits to be transmitted are allocated; and for a carrier signal with a smaller S/N, a smaller number of bits to be transmitted are allocated.
By the above allocation, the receiving side measures the S/N to prepare the bitmap which indicates the numbers of bits to be transmitted corresponding to the carrier numbers.
The receiving side informs this bitmap to the transmitting side during a training period so that both the transmitting side and the receiving side can perform the modulation/demodulation with the use of the same bitmap during normal data communication.
(4) Countermeasure Against Cross-Talk From the Time Compression Modulation ISDN Transmission (hereinafter referred to as TCM ISDN Transmission)
When there is a cross-talk due to the TCM ISDN Transmission, in the prior art, two different bitmaps are used in the ADSL modem in the transmitting side or in the receiving side so as to improve the transmission characteristic. This method of using the two bitmaps will be explained with reference to FIG. 14.
In the TCM ISDN transmission, the central office side transmits downstream data during a prior half of one cycle of a reference clock signal of 400 Hz shown in (1) of FIG. 14, in synchronization with the reference clock signal of 400 Hz; and the remote terminal side receives the downstream data and then transmits upstream data. Therefore, the ADSL modem in the central office is influenced by a Near End Cross-Talk (hereinafter referred to as NEXT) from the ISDN during the prior half of the one cycle of 400 Hz, and is influenced by a Far End Cross-Talk (hereinafter referred to as FEXT) from the upstream data of the remote terminal side ISDN.
Contrary to the central office, the ADSL modem in the remote terminal is influenced by a FEXT during a prior half of one cycle of the reference clock signal of 400 Hz, and is influenced by a NEXT during a latter half of the cycle.
If the metalic cable between the central office and the remote terminal is long, the S/N of the receiving signal to the NEXT is made smaller, and in some cases, the NEXT may be greater than the receiving signal.
In these cases, since the influence of the FEXT is not so large, in the prior art, two bitmaps are provided. One is a bitmap (DMT symbol X) for receiving signals during the NEXT period at the remote terminal. The other is a bitmap (DMT symbol Y) for receiving signals during the FEXT period at the remote terminal. During the NEXT period, in the prior art, the number of bits to be transmitted is made small so as to improve the resistance of the signals against the S/N. During the FEXT period, in the prior art, the number of bits to be transmitted is made large so as to increase the transmission capacity.
On the other hand, the time interval of one DMT symbol is usually 246 μs with a Cyclic Prefix of 16 points. Contrary to this, in the prior art, in order to conform the one DMT symbol with the TCM Cross-talk period of 400 Hz, the time interval of one DMT symbol is made to be 250 μs with a Cyclic Prefix of 20 points so that one period of the TCM Cross-talk is made to coincide with the time period of ten DMT symbols, whereby the synchronization with the TCM Cross-talk is established.
The above-mentioned prior art method of employing the two bitmaps, however, is largely different from the standard system in which only a single bitmap is employed. If two bitmaps are employed, the sequence of informing the bitmaps obtained from the S/N during a training period from the receiving side to the transmitting side must be modified, and in addition, the informing period is doubled so that the training period is increased.
In the apparatus of the central office or the remote terminal, the memory capacity must be increased in order to store the bitmaps, so that a cost problem occurs.
Further, to change the length of the Cyclic Prefix is largely different from the specification of the standard system so that the above-mentioned countermeasure against the TCM cross-talk cannot be performed in the hardware of the apparatus employing the standard system.